1. Field of the Invention
This invention relates to a process for shutoff of fuel injection during the deceleration phases of an internal combustion engine.
2. Description of the Prior Art
With electronic fuel injection systems, whether they be monopoint or multipoint, it is possible to improve average engine consumption by shutting off fuel injection during sharp deceleration phases.
This fuel injection shutoff can be triggered in response to closing of an electric switch indicating the closed position of the throttle controlling the engine air flow, detection of a threshold of the pressure prevailing in the intake manifold, detection of a predetermined limiting injection time or the like.
The fuel saving realized will be greater, the longer the injection is shutoff. Therefore, it is desirable to restart the injection system, i.e., to inject fuel again, at an engine speed that is as low as possible.
This restart is not accomplished without difficulty because it has been found that the speed drop of a no-load engine is much greater when it is not being fed fuel. Therefore, the procedure of shutoff in deceleration results in a high risk of engine stall when injection is restarted, because of what hereafter will be called the "restart speed or threshold," being too low. This phenomenon is notable after a no-load acceleration (particularly in the presence of turned power steering) or when the driver disengages the clutch after a long deceleration under load.
Although the difficulty can be encountered in multipoint injection systems (one injector per cylinder), it is greatly amplified with a monopoint type injection system comprising a single injector positioned above a butterfly placed in a body similar to that of a carburetor to control the engine air flow. Actually, for the latter there is a considerable lag between the order to resume injection and the moment when the cylinder has received sufficient fuel to assure a correct combustion.
Patent No. GB-A-2,062,295 attempts to solve to this problem by means of a process by which fuel injection is interrupted in response to detection of a deceleration, the period of rotation of the engine is measured from consecutive equal intervals or angles of rotation and the difference between the period measured from the last interval and the period measured from the preceding interval is calculated. This difference is compared with a fixed threshold to determine if the engine is in a state of slow or fast deceleration and again the fuel injection is ordered at one or other of the two fixed thresholds of engine speed depending on whether it is in slow or fast deceleration.
However, this process is not reliable because of the existence of rapid variations of the instantaneous period of the engine which are often fleeting and of a mechanical origin, such as play in the transmissions, engine suspension problems, or any other phenomenon causing instantaneous, short variations in speed. Erroneous detections of slow or fast deceleration conditions can result and be reflected either in a premature "restart" if a fast deceleration is detected in error or, on the contrary, in stalling of the engine in the opposite case. Further, the use of two fixed "restart" thresholds instead of a single one is a compromise solution that does not totally eliminate the risks of engine stalls and does not make it possible to optimize the fuel injection shutoff process in deceleration.